It is recognized that the use of fiber optics to transfer information bearing signals offers advantages over electrical transmission lines or metallic waveguides, principally due to the wide bandwidth and low loss of fiber optics. The signals of interest are electrical, and must be impressed upon a lightwave carrier before injection into a fiber optic waveguide. A detector at the destination of the fiber optic converts the optical signal back to electrical domain. The hardware that converts the electrical signal to the optical domain and then back to the electrical domain comprises, along with the fiber optic waveguide, an optical link. The performance of the link is affected by each of its components: the laser light source, the device that converts from the electrical to optical domain (which may be the laser itself), and the detector. The accuracy with which the signal is impressed upon the lightwave carrier has an impact on the performance of the optical link and is related to the linearity of the link.
Optical links are engineered to be linear for a specified range of input amplitudes (or power) and resulting output amplitudes (or power). This range constitutes the dynamic range of the system (either input or output). Factors that affect dynamic range include the laser power, the noise of the system, the linearity of the method for converting an electrical signal to the optical domain and the linearity of the detector. Methods of impressing an electrical signal onto the light carrier included either direct modulation of the laser or using an external modulator. Direct modulation of a laser is limited to bandwidths of around 1 GHz. Higher modulation frequencies require external modulators.
Methods of external modulation have been demonstrated in the prior art employing electro-absorption modulators (EAM), Mach-Zender modulators (MZM), and directional couplers (DC). An EAM can be modulated at high speeds, however its large optical attenuation and sensitivity to environmental factors (temperature) have limited its use. The MZM is very successful commercially. The transfer function of the MZM is a cosine function, and input signals are limited to a small part of the transfer function in order to form a linear analog link. The DC modulator is formed by placing two waveguides of an electro-optic material close together and applying an electrical signal to alter the coupling between the waveguides in accordance with the applied signal. The transfer function is a sin(x)/x function. Improvements to the linearity of the MZM or DC transfer function comprise the field of linearization. MZM still lacks sufficient dynamic range.
Linearity is described in the following fashion. The input signal can be represented by a sum a sinusoidal signals of some specific frequency, amplitude and phase. For each individual frequency input to the link, a perfectly linear system would produce only that frequency at the output (with altered amplitude and phase). No present method of converting an electrical signal to the optical domain is perfectly linear, and the non-linearity presents itself as frequencies in the output other that which was input. A single frequency at the input yields many frequencies in the output. The additional frequencies present in the output that were not in the input are the source of a distorted (i.e., nonlinear) output signal.
Engineering solutions allow linear optical links to be created from nonlinear components (eg cosine transfer function of the MZM) by restricting the amplitude of the input signals to a range where the inevitable distortion frequencies have a magnitude that is small. This typically means that they have a magnitude below the noise level of the optical link. The maximum allowable input signal is that which causes a nonlinear frequency component to have a power equal to the noise level of the optical link, with the minimum input signal being noise level of the system. This defines the range of input, and associated with this is an output range. Expressing the input and output ranges in terms of signal power (rather than amplitude) defines the dynamic range of the system.
Several linearization techniques have been proposed in the prior art. The basis for many of these techniques is to carefully create a non-linearity that offsets the non-linearity of either the MZM or DC. However, for signals greater than approximately 1 gigahertz electrical predistortion does not work and devices with linear electrical-to-optical transfer functions must be employed.
Typical resulting architectures take the form of multiple MZMs or DCs that are cascaded either series or parallel. U.S. Pat. Nos. 5,671,302 and 5,854,862 to Skeie, U.S. Pat. No. 5,359,680 to Riviere cascade MZMs. U.S. Pat. No. 5,031,235 to Raskin, et al employs a pair of MZMs combined with a directional coupler. These approaches in general require the optical or electrical signal to be split with some degree of precision. Electrical splits are bandwidth dependent. Moreover, electrical splits unavoidably result in a reduction of signal power. Additionally, multiple electrical biases are often required for each component MZM or DC. Theoretically the linearity has been shown to be increased with the added complexity with these techniques, however in practice reliable systems have not resulted due to either bias stability and control issues or fabrication tolerances (in the case of multi-electrode DC architectures).
Another approach in the prior art is to use a Y-fed directional coupler (constant coupling), with multiple electrodes. U.S. Pat. No. 5,309,532 to Chang is an example of this, employing a 3-section directional coupler and a single, fixed coupling segment with a single pair of electrodes connected in series with two passive sections that have separate DC biases. U.S. Pat. No. 5,230,028 uses an approach similar to U.S. Pat. No. 5,309,532.
Linearization of a DC modulator using variable coupling has been described [Laliew et al, J. of Lightwave Technology, vol. 18, no. 9, pp. 1244-1249, September 2000], however the coupling function required regions of both positive and negative coupling. Implementing 180 degree phase changes in the coupling function has presented technical challenges. The prior art lacks for a method where positive coupling can be employed without necessitating 180 degree phase changes.